USB: gadget/printer, fix sleep inside atomic

Stanse found that sleep is called inside atomic context created by
lock_printer_io spinlock in several functions. It's used in process
context only and some functions sleep inside its critical section. As
this is not allowed for spinlocks, switch it to mutex.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Craig W. Nadler <craig@nadler.us>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>